Method for reducing and/or preventing production of excessively low density polymer product during polymerization transitions

Abstract

A method for controlling a transition from an initial polymerization reaction to a target polymerization reaction in a manner that reduces significantly the amount of off-grade product having excessively low density produced during the transition, including steps of: during the transition, maintaining a first one of a concentration ratio and a feed ratio at an at least substantially constant value while implementing process changes in an effort to bring produced polymer into compliance with a target specification set and monitoring (without controlling) the other one of the concentration ratio and the feed ratio to generate first data; and during the transition, determining from the first data whether polymer having excessively low density is likely to be produced during the transition, and upon determining that polymer having excessively low density is likely to be produced during the transition, maintaining during the remaining portion of the transition the other ratio at a value that is at least substantially constant while allowing the first ratio to vary is provided.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application claims the benefit of Ser. No. 60 / 875,444, filed Dec. 18, 2006, the disclosure of which is incorporated by reference in its entirety.FIELD OF THE INVENTION[0002]The invention pertains to methods for controlling polymerization reactions (e.g., olefin polymerization reactions in fluidized-bed, gas phase reactors) to implement transitions from an initial reaction (in which a polymer product is produced to meet a first specification set) to a target reaction (in which a polymer product is produced to meet a second specification set). More particularly, the invention pertains to methods for reducing the amount of product and / or preventing production of polymer (having excessively low density) that is produced while implementing such a transition.BACKGROUND OF THE INVENTION[0003]With reference to two monomers (i.e., of a set of at least two monomers that are polymerized to produce a copolymer), the term “comonomer” is herein us...

AI Technical Summary

Benefits of technology

[0021]In a class of embodiments, both the initial polymerization reaction and the target polymerization reaction produce polyethylene in a fluidized bed reactor, the comonomer to monomer feed ratio is the ratio of the feed rate of hexene or butene (a comonomer) into the reactor to the feed rate of ethylene (a monomer) into the reactor, and the comonomer to monomer concentration ratio is the ratio of hexene or butene gas concentration to ethylene gas concentration in the reactor. Until the present invention it had not been known how to choose between control of the comonomer to monomer concentration ratio and the comonomer to monomer feed ratio to implement such transitions, to prevent occurrence of a reactor discontinuity event or resin stickiness during such transitions.

[0022]In some embodiments, the inventive control method prevents or reduces production of polymer product having excessively low density during the transition. Reduction or prevention of excessively low density polymer resin production makes the transition safer and more robust and reduces reactor outages due to shutdowns.

Problems solved by technology

Such resin chunks or sheets may fall onto the reactor's distributor plate causing impaired fluidization, and in many cases forcing a reactor shutdown (a “discontinuity” event).

Above this fusion or sintering temperature, empirical evidence suggests that such fusion or sintering leads to resin agglomeration or stickiness, which in turn can, if left unchecked, lead to formation of resin chunks or sheets and impaired fluidization.

It is known that production of polymer resin having excessively low density during resin-producing polymerization reactions in fluidized-bed, gas phase reactors is typically undesirable because such resin can be or become sticky at normal reaction temperatures, and that such resin stickiness during the reaction can cause a reactor discontinuity event (e.g., due to sheeting or chunking as mentioned above).

Unless the transition is implemented appropriately, such off-grade product may become sticky under the conditions (including temperature) present during the transition, and agglomeration or sheeting (on the reactor wall or dome) as well as product discharge problems can result.

Due to such changes in thermodynamics and kinetics, it is typically not possible to know the precise value at which the concentration of comonomer in the reactor or the feed rate of comonomer flowing into the reactor should be maintained, either during or at the end of the transition.

Often however, the correct ratio is not known, and as a result the conventionally controlled transition produces excessively low density polymer which often causes various associated problems and even undesired reactor shut down (e.g., due to sheeting caused by resin stickiness).

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